There are known endoscopes which include a plurality of illumination optical systems provided in an insertion portion at a distal end thereof, the illumination optical system having an illumination lens and a light guide fiber for guiding an illuminating light to the lens. When an endoscope is used in order to observe the inside of a body cavity, an insertion portion thereof is inserted into the body cavity, an illuminating light emitted from a light source is guided by a light guide fiber to the insertion portion, the guided illuminating light is dispersed by a lens to illuminate a site to be observed.
As such endoscopes, Japanese Unexamined Patent Application, Publication No. Hei10-288742 (PTL 1) discloses an endoscope provided with a plurality of illumination optical systems with one concave lens, in which each illumination optical system illuminates an inserting direction and a side of a distal end portion thereof respectively. Japanese Unexamined Patent Application, Publication No. 2006-72098 (PTL 2) discloses an endoscope in which an illumination optical system comprising one or three convex lenses and an illumination optical system comprising one aspheric convex lens are used in combination, as appropriate, to illuminate a site to be observed.